A purpose of this invention is to provide a stable hydroborating and reducing agent in which the donor residue can be washed away with water.
One possibility considered was the use of amines as the donor molecule. However, the amine-borane derivatives examined are too stable to permit hydroboration and reduction under mild conditions, such as represented by the following reaction: EQU 3RCH.dbd.CH.sub.2 +H.sub.3 B:NEt.sub.3 25.degree. C. No reaction
wherein R represents an organic group and Et represents ethyl.
It appeared that the disadvantage of tetrahydrofuran might be solved by using dioxane. The 6-membered ring in dioxane is less readily ruptured than the 5-membered ring in tetrahydrofuran. However, diborane does not dissolve in dioxane in the manner in which it dissolves in tetrahydrofuran to form an addition compound. This can be illustrated as follows: ##STR3##
Evidently the presence of the second oxygen atom in the dioxane ring weakens the donor properties of the first oxygen atom so that coordination with borane, sufficiently strong so as to dissociate the diborane dimer, does not occur.
The above result made it improbable that 1,4-thioxane having the formula: ##STR4## would serve to solve the problem. This compound also contains an oxygen atom which should operate to diminish the donor properties of the sulfur atom. Indeed, no one has ever reported examining the behavior of 1,4-thioxane toward diborane.
According to the invention, it has been discovered that diborane is readily soluble in 1,4-thioxane. Moreover, diborane added until the solution is saturated yields the novel borane addition compound, borane-1,4-thioxane as shown by the following reaction: ##STR5## This novel product is a stable liquid at room temperature and is 8.0 M in borane. On cooling, it solidifies, m.p. 11.degree.-15.degree. C.
Borane-1,4-thioxane is an excellent hydroborating and reducing agent, which is miscible with the standard solvents utilized for hydroboration, i.e. tetrahydrofuran, ethyl ether and methylene chloride, and it is partially soluble in pentane. It performs all of the functions achieved by borane-methyl sulfide.
1,4-Thioxane is much less volatile than dimethyl sulfide and its odor is far less unpleasant. Consequently, borane-1,4-thioxane is far easier to work with than borane-methyl sulfide. Even more important, 1,4-thioxane possesses a modest solubility in water (0.3 M). Consequently, it is a simple matter to remove the donor residue from the product in a solvent, such as diethyl ether, methylene chloride or pentane, merely by washing it with water.
Borane-1,4-thioxane hydroborates essentially all olefins rapidly and quantitatively, desirably in an inert liquid reaction medium, to yield a wide variety of fully or partially substituted organoboranes. Thus, it hydroborates alkenes rapidly to form the corresponding trialkylboranes in excellent yields. This reaction can be represented as follows: ##STR6## wherein R signifies an organic group, and particularly a hydrocarbon group. The reaction product can be oxidized with alkaline hydrogen peroxide to provide an alcohol in essentially quantitative yield. Alternatively, the reaction mixture can be treated with aqueous sodium hypochlorite to selectively oxidize the 1,4-thioxane component to the sulfoxide without oxidizing the organoborane component. The sulfoxide is highly soluble in water and can be readily extracted into the aqueous phase. The residual organoborane in the organic phase then can be recovered, dried, and utilized for the many transformations that organoboranes undergo, as see Brown et al, Organic Syntheses via Boranes, Wiley-Interscience, New York, N.Y., 1975.
Borane-1,4-thioxane is useful in the reduction of organic compounds. Thus, it can be used to reduce carboxylic acids to alcohols according to the following procedure: ##STR7## wherein R represents an organic group, especially a hydrocarbon group which, for example, can be an alkyl, cycloalkyl, aryl or aralkyl group.
The hydroboration-oxidation reaction with borane-1,4-thioxane exhibits the same high regio- and stereoselectivity of the earlier borane reagents used for this purpose. The following reaction illustrates such a reaction. ##STR8##
Functional groups which can be reduced by hydroboration with borane-1,4-thioxane followed by hydrolysis can be summarized as follows: aldehydes to alcohols; ketones to alcohols; lactones to glycols; amides to amines; epoxides to alcohols; esters to alcohols; carboxylic acids to alcohols; and nitriles to amines. The reaction is achieved using an effective quantity of borane-1,4-thioxane at a temperature below the decomposition temperature of the reactants or the resulting product.
The following examples are presented to further illustrate the invention.